FCC technology has long been a predominant means of producing gasoline. In an FCC process, gasoline is formed as the result of cracking heavier (i.e. higher molecular weight), less valuable hydrocarbon feed stocks such as gas oil. Although FCC is a large and complex process involving many factors, a general outline of the technology is presented here in the context of its relation to the present invention.
The FCC process generally includes a reactor that is closely coupled with a regenerator, followed by downstream hydrocarbon product separation. Hydrocarbon feed contacts catalyst in the reactor to crack the hydrocarbons down to smaller molecular weight products. During this process, the catalyst tends to accumulate coke thereon, which is burned off in the regenerator.
The heat of combustion in the regenerator typically produces a flue gas having an extremely high temperature. It is desirable to provide a power recovery device, such as expander turbine, to recover energy from these high-temperature flue gases. It is known, for example, to provide a turbine that can be coupled to an air blower to produce combustion air for the regenerator or to a generator to produce electrical power.
The FCC process results in a continuous fluidization and circulation of large amounts of catalyst having an average particle diameter of about 50 to 100 microns, equivalent in size and appearance to very fine sand. For every ton of cracked product made, approximately 5 tons of catalyst are needed, hence the considerable circulation requirements. Coupled with this need for a large inventory and recycle of catalyst with small particle diameters is the ongoing challenge to prevent this catalyst from exiting the reactor or regenerator in effluent streams.
Catalyst particles are abrasive and thus are capable of damaging and eroding components located downstream of the reactor, such as a turbine. If exposed to catalyst particles, blades of the turbine would erode and result in loss of power recovery efficiency. Moreover, small catalyst fines do not erode expander turbine blades significantly but can accumulate on the blades and casing to cause rubbing. Environmental emission regulations also necessitate removal of catalyst fines from flue gas. Therefore, it is desirable to remove catalyst particles from the regenerator flue gas.
In order to remove solid catalyst particles, cyclone separators internal to both the reactor and regenerator have conventionally been implemented. Typically, the regenerator includes first and second (or primary and secondary) stage separators for the purpose of preventing catalyst contamination of the regenerator flue gas, which is essentially the resulting combustion product of catalyst coke in air. While normally sized catalyst particles are effectively removed in the internal regenerator cyclones, fines material (generally catalyst fragments smaller than about 50 microns resulting from attrition and erosion in the harsh, abrasive reactor/regenerator environment) is substantially more difficult to separate. As a result, the FCC flue gas will usually contain a particulate concentration in the range of about two hundred to one thousand mg/Nm3. This solids level can present difficulties related to the applicable legal emissions standards and are still high enough to risk damage to the power recovery expander turbine.
A further reduction in FCC flue gas fines loading is therefore often warranted, and may be obtained from a third stage separator (TSS). The term “third” in TSS typically presumes a first stage cyclone and a second stage cyclone are used for gas-solid separation upstream of the inlet to the TSS. These cyclones are typically located in the catalyst regeneration vessel. It is possible to provide more separator devices or fewer separator devices upstream of the TSS. Hence, as used herein, the term TSS does not require that exactly two separator devices are located upstream of the TSS vessel. The TSS induces centripetal acceleration to a particle-laden gas stream to force the higher-density solids to the outer edges of a spinning vortex. To be efficient, a cyclone separator for an FCC flue gas effluent will normally contain many (perhaps hundreds) of small individual cyclones installed within a single vessel. At least one tube sheet affixing the upper and/or lower ends of the cyclones acts to distribute contaminated gas to the cyclone inlets and also to divide the region within the vessel into sections for collecting the separated gas and solid phases.
Flow enters the TSS vessel from an inlet pipe located at a top of the vessel. The cyclones are mounted in an array across the width of the interior of the vessel, vertically below the inlet pipe. Generally, the gas flows downwardly into an interior of the vessel, which has a substantially greater diameter than the inlet pipe. If undiffused, gas flow from the inlet pipe travels downwardly toward the cyclones, the stream concentrating on some of the cyclones, while other cyclones not within the direct path of the stream are not used with optimal efficiency. In order to improve efficiency, it is desirable to disperse the gas flow from the inlet. In some TSS units, it has been known to provide a screen mounted to cover the inlet, and specifically, such screens have been provided in a cylindrical shape or a generally hemispherical shape having an array of openings through which the gas flows. Examples of conventional separator units are disclosed in U.S. Pat. No. 5,690,709; U.S. Pat. No. 6,673,133; and U.S. Pat. No. 6,797,026. Although such conventional screens have operated to partially diffuse the flow, a more effective diffuser is needed in the TSS.